Amplifier



INVENTOR George LBeer's.

ATTORNEY G. L. BEERS AMPLIFIER Filed Nov. 1, 1923 Aug; 24, 1937.

a e M f w a? E" n V w w A. a. x I M 7 a 8 7 )8 M. w $4 W b 6 e Siyna/Jbam for ,4 V C Patented Aug. 24, 1937 UNITED STATES PATENT OFFICE AMPLIFIER Application November 1, 1929, Serial No. 404,012

30 Claims.

My invention relates to amplifiers and it has particular relation to amplifiers of the type comprising a plurality of thermionic tubes.

In modern radio receiving sets, wherein currents at radio frequencies are amplified in a plurality of stages before detection, it is extremely desirable that the said radio-frequency amplification stages should have band-pass characteristics. It is also desirable, when signals from distant stations are to be received, to provide means whereby the sensitivity of the system may be automatically or manually increased. Furthermore, it has been found advantageous to sacrifice a certain amount of fidelity, by narrowing the band of frequencies passed, in order that a radio receiver shall be sufiiciently selective to satisfactorily receive signals from distant stations in spite of interference from stations transmitting on frequencies close to those of the desired stations. On the other hand, when receiving strong signals from nearby stations, the sensitivity of the receiver may usually be reduced and, at the same time, a broadening of the band of frequencies passed permits more faithfulreproduction of the incoming signals by reason of the fact that the high-frequency side bands are not curtailed.

In short, it is desirable to have a receiving system wherein the selectivity automatically increases with an increase in sensitivity, and wherein a decrease in sensitivity is automatically accompanied by a decrease in selectivity or a broadening of the band of frequencies passed. Inasmuch as the variation in sensitivity of the receiving apparatus is, in general, made manifest by variations in the volume of the audiofrequency output therefrom, the phrase volume control will, hereinafter, be used as synonymous with sensitivity control. Also the term selectivity will be used when referring to changes in the band-pass characteristics of the system.

It is, accordingly, an object of my invention to provide a volume-control system, of the type described, whereby the selectivity of radio receiving apparatus shall, automatically, keep pace with changes purposely made in its sensitivity.

Another object of my invention is to provide a volume-control system that shall be especially useful in connection with receiving apparatus of the superheterodyne type.

, An amplifying system constructed according to my invention comprises a plurality of thermionic tubes interconnected through tuned net- 55 works having band-pass characteristics, and it further comprises means whereby control of the sensitivity of the system is accomplished through the application of negative potentials to the grid electrodes of the thermionic tubes. By properly correlating the tuned networks to the thermionic tubes used, and by judicious choice of the potentials applied to the said tubes, I am enabled to obtain automatic control of the peak width of the band of frequencies passed, at any point in the tuning range, in response to changes purposely made in the sensitivity of the system by manual or automatic devices.

The novel features that I consider characteristic of my invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and its method of operation, will best be understood from the following description of a specific embodiment, when read in connection with the accompanying drawing.

Figure 1 of the drawing is a diagrammatic view of a high-frequency amplifier comprising two alternative forms of my invention; and

Fig. 2 illustrates the manner in which automatic volume control may be embodied in a circuit such as disclosed in Fig. 1.

The apparatus illustrated in Fig. 1 of the drawing comprises a plurality of thermionic tubes I, 2 and 3, each tube having an anode 4, a cathode 5 and a control electrode 6.

Each of the tubes is provided with a tuned output circuit constituted by an inductor 1 and a tuning condenser 8 connected in shunt thereto.

In addition to the tuning condenser, a resistor IE may also be connected in shunt to one or more inductors 1 comprised in the output circuits. The function of the resistors will, hereinafter, be described in detail.

Each of the thermionic tubes is provided with a tuned input circuit constituted by an inductor II and a tuning condenser l2 connected in shunt thereto, the tuning of the input and output circuits and the coupling therebetween being so adjustable that the amplifier may be set to efficiently pass a band of frequencies substantially 1O kilocycles wide at any point in the tuning range, under conditions of minimum sensitivity, and a band 3 to 5 kilocycles wide, under maximum sensitivity conditions.

In the event that my invention is applied to the intermediate-frequency amplifying stages of a superheterodyne receiver the capacities of the condensers 8 and I2 and the percentage of coupling between the inductors 1 and II is so fixed, at the factory, that the amplifier has the desired band-pass characteristics with respect to the intermediate frequency chosen.

In order that oscillations shall not be generated in the several circuits, a feed-back neutralizing condenser i3 is connected, between the anode of each of the tubes and the end of the input circuit inductor ll opposite to the end connected to the grid of the said tube, as described in the patent to Rice, No. 1,334,118.

,A grid-bias-potential battery is provided, the

positive terminal of which is connected to a conductor common to the cathodes of the thermionic tubes. A potentiometer It is connected in shunt relation to the grid-biasing battery,

15 and the movable arm I! of the potentiometer is connected to an intermediate point on each of the input inductors H through a conductor l8 that is common thereto.

By properly adjusting the coupling between the output and the input circuits that constitute the coupling-network between successive tubes of the amplifier, a broad-top resonance characteristic may be obtained. If the coupling between the circuits is reduced, the selectivity of the system is increased and it may be made to approach the characteristic that would be obtained by so disposing of the circuits, separately, in successive stages of the amplifier that no inductive coupling exists between them. If the coupling is increased beyond a certain value the selectivity characteristic will become double-peaked.

If a variable resistor is so connected across either of the coupled tuned circuits that the terminal impedance can be caused to vary between a value less than the impedance of the tuned circuit and a value several times the said impedance, the peak of the selectivity curve may be varied in width in much the same manner as by altering the coupling.

An increase in terminal impedance is accompanied by a broadening of the peak of the selectivity curve. By limiting the amount of variation of the aforementioned resistor the effect upon the selectivity characteristic can be controlled.

In the amplifier illustrated in the drawing, each of the several tuned output circuits is effectively shunted by the plate impedance of the thermionic tube to which it is connected. As the volume-control potentiometer is shifted, to vary the negative grid potential applied the grid electrodes of the several thermionic devices, the plate impedance of each device may be caused to vary from its normal value which, for thermionic tubes of the usual types, is about 10,000 ohms, to many times this value. Accordingly, when the system is adjusted for maximum amplification, by decreasing the negative bias applied to the grid electrodes, the tube impedance effectively shunting each of the output-inductors I is, comparatively speaking, low. When the volume control is adjusted for minimum sensitivity by increasing the negative bias, the tube impedance 5 shunting each of the output-inductors 7 is greatly increased.

It is thus evident that, in my improved amplifying system, when the sensitivity thereof is altered by changing the plate-impedance of the thermionic tubes, the width of the peak of the resonance curve is correspondingly altered. In other words, by my novel combination of bias volume control and coupled tuned circuits, I am able, automatically, to obtain an increase in selectivity with increase in sensitivity and a decrease in selectivity, to provide improved fidelity of reproduction, with decreased sensitivity.

If the impedance across the output circuits of the tubes is to be prevented from increasing to such high value that the characteristic becomes double-peaked, one or more of the output circuits may be provided with shunting resistors, such as the resistor I 0, which resistor, in a successful commercial embodiment of my invention, is given a magnitude of the order of 40,000 ohms.

It will be apparent that I have, by my invention, provided an improved volume-control system whereby a radio receiving set or the like may be so automatically controlled that its selectivity always keeps pace with its sensitivity. In other words, a receiving set comprising my invention may be adjusted for maximum sensitivity to receive distant stations and, at the same time, the selectivity of the set is increased in order that interference shall be minimized. When receiving strong signals from local stations, at which time minimum sensitivity is required, my improved system causes a sufiicient broadening of the tuning of the amplifier to improve the fidelity of reproduction.

The beneficial efiects of my invention become increasingly apparent as the number of amplifying stages is increased, by reason of the fact that the selectivity and amplification obtainable by an amplifier is a geometric function of the number of stages.

The automatic change in the selectivity characteristic of a receiver, with volume-control, is particularly advantageous in the event that the receiver is equipped with automatic volume-control devices of the type disclosed in my copending application, Serial No. 212,791, filed August 13, 1927, now Patent No. 1,960,723 of May 29, 1934, and assigned to the Westinghouse Electric 8: Manufacturing Company. In such receiver, when modified according to my present invention, the fading of the signals being received is automatically accompanied by a sharpening of the selectivity characteristic, thus improving the adjacent channel selectivity and reducing background noise.

The circuit of Fig. 2 has been based upon the teachings of applicants patent cited above. The electron discharge device 20 constitutes the automatic volume control tube, having anode 22, cathode 24 and grid electrodes 26. The input circuit to the automatic volume control tube comprises a resistor 28 to which the grid electrode is adjustably connected, and an adjustable source of bias voltage in the form. of a potentiometer 30 connected across the terminals of the battery 32, the lower end of the aforementioned resistor 28 being adjustably connected to the potentiometer.

The resistor 28 is coupled to a signal source for automatic volume control operation, which, according to customary practice, may comprise a coupling to the input circuit of a detector stage of a receiver through a blocking condenser 34, or it may be otherwise coupled to the output circuit of one of the high frequency amplifiers of the receiver, all of which different modes of connection are disclosed in applicants patent referred to.

In the output circuit of the automatic volume control tube, there is included a source of anode potential which may constitute a battery 36 or any other suitable source, and in series therewith, a high resistance element l6, across which will be developed the automatic volume control poten- Cal circuit of Fig. l, the automatic volume control resistor It can take the place of the resistor It of the circuit of Fig. 1 and its manually adjustable contact IT. The battery M of Fig. 1 will no longer be necessary, and will accordingly be dispensed with. When utilized in place of the resistance It in the circuit of Fig. l, the upper lead l8 will correspond to the lead designated by the reference numeral IS in the circuit of Fig. 1 and the lower lead l5 will correspond to the wire Although I have selected a specific type of circuit for purposes of illustration, many modifications thereof will be apparent to those skilled in the art to which it pertains. My invention, therefore, is not to be limited except insofar as is necessitated by the prior art or by the spirit of the appended claims.

I claim as my invention:-

I. In a tuned amplifier designed to efliciently amplify a band of frequencies, means for decreasing the sensitivity and increasing the impedance of the amplifier, and means whereby a decrease in the sensitivity of the said amplifier is prevented from being accompanied by undesirable broadening of the tuning thereof.

2. In an amplifier, a plurality of thermionic devices, and a transformer having tuned primary and secondary windings interposed between a pair of said devices to constitute a band-pass filter, the impedance of the tuned circuit, comprising the primary winding, being at least equal to the impedance of the tube to which said primary winding is connected, when the amplifier is adjusted for maximum sensitivity, and the coupling between the primary and secondary windings being adjusted in said maximum-sensitivity condition to be less than the value which would give a band-pass characteristic for passing a frequency band of maximum width.

3. In an amplifier, an electric discharge device, an output circuit for said device comprising a tuned circuit, a second electric discharge device, an input circuit for said second device comprising a tuned circuit, said output and input circuits being so coupled together as to be highly selective when the impedance of said first device is at its normal value, and means for adjusting the impedance of said first device, whereby the width of the frequency band passed by said coupling is changed.

4. In an amplifier, an electric discharge device comprising cathode and anode elements and at least one grid, and output circuit for said device comprising a tuned circuit, a second electric discharge device comprising cathode and anode elements and at least one grid, an input circuit for said second device comprising a tuned circuit, said output and input circuits being so coupled together as to be highly selective when the impedance of said first device is at its normal value and volume-control means for adjust ing the potential applied to a grid of said first device whereby the sensitivity of said first device and the selectivity of said coupling are concurrently decreased.

5. In an amplifier, an electric discharge device comprising cathode and anode elements and a control element, an output circuit for said device comprising a tuned circuit, a second electric discharge device comprising cathode and anode elements and a control element, an input circuit for said second device comprising a tuned circuit, said output and input circuits being so coupled as to efliciently amplify only a portion of the ten kilocycle side band frequencies when the imedance of said first device is at its normal value, and volume-control means comprising means for increasing the negative bias on said control element of said first device whereby the sensitivity of said first device and the selectivity of said coupling are concurrently decreased.

6. In an amplifier, an electric discharge device comprising cathode and anode elements and a control electrode, an output circuit for said device comprising a transformer primary having a condenser connected thereacross, a second electric discharge device comprising cathode and anode elements and a control element, an input circuit for said second device comprising a transformer secondary coupled to said primary and having a condenser connected thereacross, said output and input circuits being so coupled and tuned as to form a band-pass filter efficiently amplify only a portion of the ten kilocycle side band frequencies when the impedance of said first device is at its normal value, and volume-control means for increasing the negative bias of said control element of said first tube whereby the sensitivity of said first device and the selectivity of said filter are concurrently decreased.

7. In an amplifier, an electric discharge device, an output circuit for said device comprising a transformer primary having a condenser con-.

said first device whereby the selectivity of said coupling is decreased.

8. In an amplifier, an electric discharge device, an output circuit for said device comprising a transformer primary having a condenser connected thereacross, a second electric dischargedevice, an input circuit for said second device comprising a transformer secondary coupled to said primary and having a condenser connected thereacross, said output and input circuits being so coupled and tuned as to form a band-pass filter, and volume-control means for decreasing the sensitivity and increasing the plate impedance of said first device whereby the selectivity of said filter is decreased, and a resistor shunted across said primary and having a resistance value greater than the minimum plate impedance.

9. In an amplifier, a thermionic device, an output circuit for said device comprising a transformer primary having a condenser connected thereacross, a second thermionic device, an input circuit for said second device comprising a transformer secondary coupled'to said primary and output and input circuits being so coupled and tuned as to form a band-pass filter, and volumecontrol means for decreasing the sensitivity and increasing the plate impedance of said first device whereby the selectivity of said filter is decreased, said plate impedance having a value such that the terminal impedance of said primary is variable between a value less than the impedance of said tuned output circuit and a value greater than the impedance of said tuned output circuit.

10. In an amplifier, a plurality of electric discharge devices, adjacent devices being coupled through a transformer having tuned primary and tuned secondary windings, volume-control means for adjusting the negative bias of the input electrodes of said devices in accordance with the volume desired, said coupling being so adjusted as to eificiently amplify only a portion of the ten kilocycle side band frequencies when the impedance of one of said devices is low.

11. In an amplifier, a plurality of electric discharge devices, adjacent devices being coupled through a transformer having tuned primary and tuned secondary windings which act as a bandpass filter, means for decreasing the sensitivity and increasing the impedance of the one of said devices having its output connected to said primary, and means comprising an impedance unit connected across said primary for preventing the tuning of said filter from being broadened more than a predetermined amount when the impedance of said devices is increased.

12. In a system of the radio receiving type wherein a pair of electric discharge devices are coupled through a pair of tuned circuits, and in which volume control is obtained by varying the grid bias potential on one of said devices, the method which comprises placing one of said devices at its minimum operating impedance condition, then adjusting the degree of coupling between said devices until only a portion of the ten kilocycle side band frequencies is efficiently amplified.

13. The method of operating a radio receiver including a radio frequency sensitivity control .;,5 device and a tone control device which comprises reproducing the higher audio frequencies in inverse proportion to the radio frequency sensitivity of the radio receiver.

14. In the operation of a modulated carrier wave receiver, the method which comprises reproducing substantially all received audio frequencies at approximately their relative values when the sensitivity of the receiver is reduced to its lower limit, and automatically effecting a progressive suppression of the higher audio frequencies as the sensitivity of the receiver is increased.

15. In a modulated carrier wave receiver, the combination with means for controlling the sensitivity of the receiver, of means coupled thereto for simultaneously shifting the upper limit of the band of audio frequency transmission toward a lower limit as the sensitivity of the receiver is increased.

16. In the operation of a modulated carrier 5 wave receiver, the method of maintaining a high signal to noise ratio which comprises automatically decreasing the width of the band of audio frequency transmission as the sensitivity of the receiver is increased.

17. In the operation of an amplifier adapted to amplify a band of frequencies, the method of simultaneously controlling the intensity and tonal characteristics of the energy passing through the amplifier which comprises automati- Z5 cally decreasing the width of the band of frequencies amplified by the amplifier as the amp1ification factor thereof is increased and increasing the width of the band of frequencies amplified as the amplification factor is decreased.

18. The method of operating a radio receiver means, means for simultaneously operating said device to reproduce the higher audio frequencies in inverse proportion to the radio frequency sensitivity of the radio receiver.

20. In the operation of a modulated carrier wave receiver, the method which comprises auto-;

matically varying the width of the band of reproduced audio frequencies inversely as the magnitude of received radio frequency signal energy.

21. In the operation of an amplifier adapted "to amplify a band of frequencies, the method of simultaneously controlling the intensity and tonal characteristics of the energy passing through the amplifier which comprises decreasing the width of the band of frequencies amplified by the amplifier as the amplification factor there-n:

of is increased and increasing the width of the band of frequencies amplified as the amplification factor is decreased.

22. In a tunable radio receiver, a plurality of cascade-connected electronic devices, means for automatically correlating the gain in at least one of said devices to incoming signal amplitude and means for automatically causing the overall fidclity of the receiver to vary inversely with changes in gain, whereby back-ground noise may be minimized during the operation of tuning the receiver.

23. In a selective amplification system, the method of controlling the condition of operation which comprises, varying the sensitivity in response to changes in the amplitude of received signals, and simultaneously varying in the same direction the selectivity of said system in response to said changes in signal amplitude.

24. In a selective amplification system, the method of controlling the condition of operation which comprises, varying the sensitivity in response to changes in the amplitude of received signals, and simultaneously varying in the opposite direction the fidelity of said system in response to said changes in signal amplitude.

25. In a selective amplification system, the method of controlling the condition of operation which comprises, varying the sensitivity in response to changes in the amplitude of received signals, and simultaneously varying the characteristics of said system in such manner as to suppress background noise in response to a decrease in the amplitude in the received signal.

26. In a carrier frequency amplifier, a plurality of electronic tubes, each having an anode, cathode and at least one other auxiliary electrode, a source of signal energy, coupling means between said source of signal energy and the tubes and between adjacent tubes, said coupling means including circuits variably tunable to signal energy over a range of tuning, said tubes and coupling means being so constructed and connected that said tube and coupling network operates to amplfy high frequency currents and to provide an impedance in shunt with one or more of said variably tuned circuits which under some conditions of operation is less than the resonant impedance of such last-mentioned tuned circuit, adjustable means for impressing a negative bias on the auxiliary electrodes of at least certain of said tubes, said tubes and coupling means being further so constructed and arranged that increase in negative bias on the auxiliary electrodes simultaneously decreases the sensitivity and selectivity of the electronic tube amplifying net- Work.

2'7. In a carrier frequency amplifier, a plurality of electronic tubes, each having an anode, cathode and at least one other auxiliary electrode, means for coupling a plurality of tubes together intoa tube and coupling network, said coupling means including at least one circuit variably tunable to signal energy over a range of tuning, said tubes and coupling means being so constructed and connected that said tube and coupling network operates to amplify high frequency currents and to provide an impedance in shunt with one or more of said variably tuned circuits which under some conditions of operation is less than the resonant impedance of such last-mentioned tuned circuit, adjustable means for impressing a varying potential on the auxiliary electrodes of at least certain of said tubes, said tubes and coupling means being further so constructed that change in potential on the auxiliary electrodes simultaneously changes the sensitivity and selectivity of the electronic tube amplifying network in the same sense.

28. In a carrier frequency amplifier, a plurality of electronic tubes, each having an anode, cathode and at least one other auxiliary electrode, means for coupling a plurality of tubes together into a tube and coupling network, said coupling means including at least one circuit variably tunable to signal energy over a range of tuning, adjustable means for impressing a varying potential on the auxiliary electrodes of at least certain of said tubes, said tubes and coupling means being further so constructed that increase or decrease in potential on the auxiliary electrodes simultaneously changes the sensitivity and selectivity of the electronic tube amplifying network in the same sense.

29. In a carrier frequency amplifier, a plurality of electronic tubes, each having an anode, cathode and at least one other auxiliary electrode, means for coupling a plurality of tubes together into an amplifying tube and coupling network, adjustable means for impressing a negative biasing potential on the auxiliary electrodes of at least certain of said tubes, said coupling means comprising like tuned circuits coupled to give a band pass effect and one of said tubes being effectively shunted across one of said tuned circuits and having an impedance under conditions of relatively low negative bias on its auxiliary electrode less than the resonant impedance of the tuned circuit so that change in potential on the auxiliary electrode simultaneously changes the sensitivity and selectivity of the electronic tube amplifying network in the same sense.

30. In a carrier frequency amplifier, a plu, rality of electronic tubes, each having an anode, cathode and at least one other auxiliary electrode, means for coupling a plurality of tubes together into an amplifying tube and coupling network, adjustable means for impressing a negative biasing potential on the auxiliary electrode of at least one of said tubes, said coupling means comprising like tuned circuits coupled to give a band pass effect with approximately critical coupling at maximum negative biasing potential on the auxiliary electrode and one of said tubes being effectively shunted across one of said tuned circuits and having an impedance under conditions of minimum negative bias on its auxiliary electrode less than the resonant impedance of the tuned circuit so that change in potential on the auxiliary electrode simultaneously changes the sensitivity and selectivity of the electronic tube amplifying network in the same sense.

GEORGE L. BEERS. 

